I am not a doctor and am not giving medical advice.
Follow the "Body Blues" program to increase natural seratonin production.
www.thebodyblues.com I got the book from the library, it is called When Your Body Gets the Blues. It is very simple. You're already getting natural light and exercise, so you're most of the way there! You can buy the supplements separately or they sell a formula that has all them in one pill.
Learn some basic relaxation yoga or at least some simple stretches. Stress control is important.
Start a progesterone/estrogen cream. I am currently using Progesta-Care Plus. Our homones plummet after birth. I have learned that I am very sensitive to hormone fluctuations. (Since you are nursing you should run this past your midwife. Is your supply well-established? Personally, I don't think it should be a big problem, as many mothers nurse through pregnancy and their hormone levels are significantly higher than anyone would have using the cream. You can also buy progesterone only cream. The mini pill is progesterone only and is prescribed to nursing mothers. This would be a much lower dose.)
Take prescription iron if you are prone to anemia; I get very depressed when I have low iron (under 11), even if I am not clinically anemic. I do not get enough iron in my diet and/or I do not absorb it well. I like Chromagen Forte the best. If you cannot get a prescription and have a history of low iron then I would buy an OTC iron pill like Slow FE.
Take a good EFA for women, such as Barlean's Essential Woman (I found it at my HFS). This will also promote hormonal balance.
Journal, preferably daily.
Take long hot baths with essential oils; I use a mixture of geranium, lavender, and bergamot, which is good for depression. I used to carefully measure and premix it, now I just put in severeal drop from each bottle as the water runs.
Go to group therapy if there is one available. If not, I would establish a relationship with a therapist and go at least monthly, more often if symptoms worsen. If I could not afford this or did not have insurance I would look into free counseling from a local university.
Make sure you have good friends to get together with regularly. My "PPD" lasted from the birth of my first son to when my second was almost 5 (obviously it was far more severe in the first year after birth, but I never seemed to be able to shake it). Having a regular weekly rhythm, meeting like-minded people, and getting together regularly has been a big part of my finally becoming depression free.
Eat a whole foods diet. DH might have to cook, so I would make sure he knew the basics like how to use the pressure cooker, crock pot, and rice maker. I would not hesitate to buy "convenience" foods like canned beans, pre-cut salad greens, frozen brown rice, sliced veggies (from Trader Joes), etc.
Hire a housecleaner if you can afford it. Basically, do what you can to lighten your load.
Have an open mind about using medications. If I was doing all of the above and still needed help I would not hesitate to use it.
Good luck!
Kimberly